1. Field of the Invention
The present invention relates to a process for producing metal foams and to foamed metal bodies obtained using the process of the present invention.
2. Background of the Invention
The prior art for producing metal foams includes essentially five principle procedures. These five principle procedures include:
1. Compacting metal powders together with suitable blowing agents and heating the compressed green bodies obtained in this way to temperatures above the liquidus temperature of the metal matrix and above the decomposition temperature of the blowing agent used;
2. Dissolving or injecting blowing gases in/into metal melts;
3. Stirring blowing agents into metal melts;
4. Sintering hollow metal spheres; and
5. Infiltrating metal melts into porous bodies which are removed after solidification of the melt.
Regarding prior art procedure 1: DE 197 44 300 A is concerned with the production and use of porous light metal parts or light metal alloy parts, in which the bodies pressed from a powder mixture (light metal alloy or Al alloy and blowing agent) are heated in a heatable closed vessel provided with inlet and outlet openings to temperatures above the decomposition temperature of the blowing agent and/or melting point of the metal or alloy.
Regarding prior art procedure 2: JP 03017236 A describes a process for producing metal articles containing voids by dissolving gases in a metal melt and then inducing foaming by sudden reduction of pressure. Cooling of the melt stabilizes the foam obtained in this way.
Wo 92/21457 teaches the production of Al foam or Al alloy foam by injecting gas under the surface of a molten metal, with abrasive materials, e.g. SiC, ZrO2, etc., serving as stabilizers.
Regarding prior art procedure 3: According to JP 09241780 A, metallic foams are obtained by means of controlled liberation of blowing gases by first melting the metals at temperatures below the decomposition temperature of the blowing agent used. Subsequent dispersion of the blowing agent in the molten metal and heating the matrix above the temperature required for liberation of blowing gases forms a metal foam.
Regarding prior art procedure 4: Ultralight Ti-6Al-4V hollow sphere foams are produced from hollow metallic spheres derived from the thermal decomposition of hydrogenated Ti-6Al-4V hollow spheres at 600xc2x0 C. which are then sintered at temperatures of xe2x89xa71000xc2x0 C. (Synth./Process. Lightweight Met. Mater. II, Proc. Symp. 2nd (1997), 289-300).
Regarding prior art procedure 5: After infiltration of molten aluminum into a porous filler, foamed aluminum is obtained by removal of the filler from the solidified metal (Zhuzao Bianjibu (1997) (2) 1-4; ZHUZET, ISSN: 1001-4977).
On examination of the prior art, it can be seen that the processes which provide a precompacted green body containing blowing agents are complicated and expensive and are not suitable for mass production of goods. Moreover, in all the above processes, the desired temperature difference between the melting point of the metal to be foamed and the decomposition temperature of the blowing agent used should be very small, otherwise undesirable decomposition of the blowing agent takes place during compaction or later in the melting phase.
Analogously, these considerations also apply to the introduction of blowing agents into metal melts.
The sintering of preformed hollow spheres to give a metallic foam is at most of academic interest, since the production of the hollow spheres requires a complicated process technology.
The infiltration technique in which the porous filler has to be painstakingly removed from the foam matrix can be evaluated similarly.
The dissolution or injection of blowing gases in/into metal melts is not suitable for the manufacture of workpieces having a shape close to the final shape, since a system consisting of the melt with occluded gas bubbles is not sufficiently stable over time to be able to be processed in moulds.
In view of the above backgrounds, it is an object of the present invention to provide a simple process for the production of metal foams which is at the same time suitable for mass production and allows the production of parts having a shape close to the final shape at little cost and is based on the use of solid, gas-generating blowing agents.
It has surprisingly been found that the production of metallic foams requires neither compacted green bodies provided with blowing agents nor the introduction of blowing agents into molten metals, if particular process engineering-boundary conditions are adhered to.
In the simplest embodiment of the process of the present invention, a porous metal body can be easily produced by mixing a small amount of a gas-generating blowing agent with a powdered metal to be foamed and heating this mixture quickly. The process of the present invention provides a porous metal body.
The abovementioned object is thus achieved, in a first embodiment, by a process for producing metal foams, which comprises the steps of: mixing one or more metal powders, optionally combined with particulate metallic or nonmetallic, e.g., mineral, additives of varying particle size, with a gas-liberating blowing agent; optionally, preheating the mixture in an open or closed mold to a temperature below the decomposition temperature of the blowing agent; subsequently heating the mixture to a temperature above the melting point of the lowest-melting metal in such a way that the temperature rises from the equilibrium decomposition temperature of the blowing agent to the melting point of the metal in a time which is shorter than the time required to reach the equilibrium state in the blowing agent/blowing gas system at this temperature; and cooling the resulting metal foam to a temperature below the melting point of the lowest-melting metal in the metal mixture.